Behavior at the species boundary: the overlooked role of male mate choice in a sexually dimorphic species

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Biological Sciences


Biological Sciences

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Behavioral isolation is a powerful barrier to gene flow and is often considered a major component in the maintenance of species boundaries. Empirical support for mating preferences as an isolating barrier exists across a wide variety of taxa. In monomorphic or unornamented species, such as Lepidoptera (moths and butterflies) and Drosophila, male preferences for conspecific female traits prevent interbreeding between species. However, in sexually dimorphic species, the majority of studies focus on female mate choice for conspecific male signals, and the role of male mate choice in behavioral isolation has not been widely considered. Increasing evidence that males of many species exhibit mate choice, even those in sexually dimorphic species with traditional sex roles, suggests that the process of behavioral isolation via male mate choice should be addressed. Here, I address fundamental questions about the maintenance of species boundaries via male mate choice in a group of diverse freshwater fishes called darters (Percidae: Etheostoma). My results suggest that male mate choice for conspecific females reduces gene flow between the focal species, Etheostoma zonale and Etheostoma barrenense, with male mate choice contributing as much as female mate choice to total behavioral isolation. I also find that male and female preferences appear to be shaped by similar evolutionary mechanisms, with evidence that selection against hybrids strengthens both male and female preferences upon secondary contact. Interestingly, however, I find that the color pattern elements that promote assortative mating differ between males and females, meaning that diversification of male and female traits may follow independent evolutionary paths. Taken together, my results suggest that male behavior is likely to be underappreciated in the maintenance of species boundaries for sexually dimorphic species characterized by elaborate male ornamentation.